1. Field of the Invention
The present invention relates to white balance adjustment devices particularly suitable for television cameras, video cameras, electronic still cameras, and the like.
2. Description of the Related Art
White balance adjustment devices of cameras are well known. FIG. 1, for example, shows a well-known white balance adjustment device which is used in a color camera. Imaging unit 403 is an element which converts light into electrical signals. The imaging unit is typically a charge-coupled (CCD) device, a metal oxide semiconductor (MOS) device, or other form of solid-state imaging device. A circuit samples the output signals from imaging unit 403 at a predetermined rate. In particular, light rays are received from a photographic subject (not shown) via the optical member 401 and stop member 402. Respective R (red), G (green), and B (blue) signals are output from the imaging unit 403. The G output signals are transmitted directly to a signal processor 405; the R output and the B output are transmitted to the signal processor 405 via respective white balance variable amplifier circuits 404R and 404B controlled by means of a control unit 410 according to the output of a color measurement unit 430. Typically, as the light source varies, the levels of the R and B signals are adjusted, usually so that there is a natural color balance, based on the G signal level. The signal processor 405 performs the adjustment of gain, set-up level, gamma and knee, etc., and transmits image signals to a recording unit 406.
The prior art white balance adjustments which are made by means of variable amplifier circuits 404R and 404B will next be described. Light, received by the color measurement unit 430 from the camera surroundings, is passed through a diffusing plate 431 to a R red light receiving element 432R, to a G green light receiving element 432G, and to a B blue light receiving element 432B, and from the respective light receiving elements there are output signal currents corresponding to the red component (Rb), green component (Gb) and blue component (Bb) of the incident light. After these signal currents have been converted into logRb, logGb and logBb signals by respective logarithmic compression by a logarithmic compression circuit 433, log (Rb/Gb) and log (Bb/Gb) are calculated in subtraction circuits 434 and 435, and are furthermore converted into Rb/Gb, Bb/Gb by antilog conversion in the expansion circuit 436. After conversion into voltage signals by the current/voltage converter circuit 437, they are converted into digital signals by the A/D converter 409 and are output to control unit 410.
In the case that the auto white balance mode is selected by means of a switch 421, the gain of the variable amplifier circuits 404R and 404B corresponding to the measured color results Rb/Gb, Bb/Gb is "looked up" or retrieved from a previously prepared control table 408A for auto white balance use, and is set in the respective amplifier circuits 404R and 404B.
If the auto white balance mode is not selected via switch 421 and, alternatively, the white balance mode is selected to be in a "fine weather," "cloudy," "fluorescent lamp," or "tungsten lamp" mode via corresponding switches 422-425 (i.e., a manual white balance mode), the gain corresponding to the selected light source of the manual mode is looked up from a previously prepared control table 408M for manual whim balance use, and is set in the respective amplifier circuits 404R and 404B.
While prior art white balance adjustment devices of the type discussed above have been in use for some time, in recent years various new light sources have emerged including, but not limited to, color electric bulbs, white color emitters, daylight white color emitters, daylight color emitters, and various fluorescent lighting arrangements and lamps. Such new lighting and illumination sources present problems for prior art white balance adjustment devices. For example, photographic picture coloration is often too red or too blue, depending on the kind of fluorescent lamp which is used as an illumination source. This problem results with prior art white balance adjustment devices since such devices use the same control values and parameters for white balance adjustment regardless of the fluorescent lighting source actually being used to illuminate a subject.
In order to solve the aforementioned problems, selection switches were arranged for electric bulb color, white color, daylight white color, daylight color and the like in the fluorescent lamp manual mode, and the photographer, selecting according to the kind of fluorescent lamp, for example, could make an adjustment for the optimum color balance. However, it is difficult for photographers to determine exactly the kind of fluorescent lamp being used by mere visual inspection during use of a camera, for example.
The present invention solves the aforementioned and numerous other problems associated with white balance adjustment in photographic/video settings.